Bernie/Mamdani are not extreme left. Further left than liberals for sure, but no where close to extreme left.
And let's be clear: maybe NYC needs that, but pretending that he represents anything in the US liberal mainstream is a joke.
It's not communism to support public-owned public services, even public-owned competitive services where no commercial competition exists. Capitalism in tight spaces, creates monopolies and market quirks that harm consumers, harm renters, harms tourists. These harms are toxic to NYC. He's right to address them.
*To expand, Mamdani is moderate left. Extreme left, at least here in the states, would be actual communists and anarchists.